Add ONNX module into the CI for testing ONNX export

[thiagocrepaldi]

I am with Microsoft and have been working on adding support to ONNX export (with and without caffe2) #4120 and #4153 to detectron, so that it can run on ONNX Runtime. It seems several discussions are open here already.

If we have ONNX installed on CI environment, we can prevent future breaks. For example, #4132 breaks the proposed ONX export above and it wouldn't be caught by CI

Is there a particular reason not to have ONNX on CI?

[thiagocrepaldi]

#4190 implements this change, have the decision being made approving it